This invention relates to thin walled polyurethane elastic articles and to a method of making them.
Many devices for biomedical applications require the use of soft thin walled elastic articles. Examples of such articles are gloves for surgical operations and examinations under clean room conditions, and balloons for catheters and condoms. Natural rubber from latex is currently the material of choice for these types of applications due to its outstanding balance of mechanical properties. Typical properties are high elongation at break (800-900%), low modulus [a modulus at 100% elongation of 0.7-0.9 MPa], acceptable tensile strength (20-35 MPa) and a low degree of creep. Natural rubber does, however, have the drawback that the presence therein of proteins and other undesirable compounds, such as vulcanisation accelerator residues, can lead to human allergic reactions if these compounds are leached from the rubber network during use. Another potential hazard is the formation of nitrosamines which are suspect carcinogens. It would therefore be beneficial to replace natural rubber with a synthetic elastomer for the fabrication of articles for biomedical applications.
Polyurethanes have been used to fabricate thin walled elastic articles, usually by dip coating from organic solvent based solutions. However, these solutions are usually quite viscous, due to the high molecular weight of the polymers, and this often presents problems with processing. The molecular weight of a polyurethane elastomer is a key parameter to performance, particularly in regard to the physical properties of the material which will be poor if a suitably high molecular weight is not attained.
Quite apart from these general problems of using polyurethanes to fabricate thin walled elastic articles, there are further difficulties in using them as a replacement for natural rubber. Whilst, in general, polyurethane films can be made of good tensile strength (e.g. 30 to 60 MPa) and moderate elongation (450-650%), these known materials are much harder than natural rubber and will normally have a significantly higher modulus at 100% extension (hereinafter xe2x80x9cS100xe2x80x9d), for example of at least 2.2 and often much higher. Attempts to make softer polyurethanes have resulted in a lower modulus, but tensile strength and elongation at break have also been reduced and, most importantly, there has been an unacceptable loss in elasticity. Thus, it has not been possible to date to provide a polyurethane dipped film whose physical properties have been close to those of natural rubber.
European patent application 0741152 describes aqueous polyurethane dispersions based on polyether polyols of low unsaturation or monol content. The use of such polyols having an unsaturation of less than 0.02 meq/g polyol and preferably less than 0.01 meq/g polyol, is described as providing polyurethane dispersions of better properties. For example, substitution of the low unsaturation polyols for conventional polyols is shown to give films (cast on glass plates) having higher tensile strength, higher 100% and 300% moduli, and lower ultimate elongation.
European patent 0781791 also describes the use of low unsaturation copolymer polyols for making polyurethane elastomers. The unsaturation is 0.06 meq KOH/g or less, and the polyols are polyoxyalkylene polyether block copolymers. A principle use of the polyurethanes is for sealants and adhesives.
We have now found that by using certain low unsaturation polyols in a limited range of polyurethane polymers, it is possible to make soft thin-walled elastic polyurethane articles with physical properties very similar to such articles made of natural rubber.
In one aspect, the invention provides a soft, thin-walled elastic article made of a linear polyurethane having physical properties close to those of natural rubber wherein the article is made of a linear polyurethane which comprises an xcex1,xcfx89-dihydroxy polyol selected from poly(propylene glycol)s, said polyol containing no more than 0.01 milliequivalents unsaturation per gram; an aliphatic diisocyanate; and a chain extender; said polyurethane having a ratio of hard:soft segments from 20:80 to 40:60, a number average molecular weight (Mn) of from 90 to 150 kg/mole and a ratio of average molecular weight (Mw) to Mn of from 1.2 to 2.2; and wherein the said film has an S100 of less than 2.0 MPa, an elongation at break of at least 800% and a tensile strength of above 15 MPa.
In another aspect, the invention provides a method of making a soft, thin walled glove or condom of a linear polyurethane have properties close to those of natural rubber, which comprises dipping a former in an organic solution or aqueous dispersion of a linear polyurethane which comprises xcex1,xcfx89-dihydroxy polyol selected from poly(propylene glycol)s, said polyol containing no more than 0.01 milliequivalents unsaturation per gram; an aliphatic diisocyanate; and a chain extender; said polyurethane having a ratio of hard:soft segments from 20:80 to 40:60; a number-average molecular weight (Mn) of from 90 to 150 kg/mole and a ratio of average molecular weight (Mw) to Mn of from 1.2 to 2.2; drying the coated former and removing the glove or condom therefrom.
As used herein, the term xe2x80x9cpolyurethanexe2x80x9d includes xe2x80x9cpolyurethane-ureaxe2x80x9d.
The polyurethanes of the invention are linear and are made from one or more isocyanates, one or more polyols and one or more chain extenders. The polyols are polyether polyols based on propylene glycol and thus are xcex1, xcfx89-dihydroxy polyols. They preferably have a molecular weight of from 400 to 12000 daltons.
The polyurethanes of the invention can be made either by the so-called xe2x80x9cone-shotxe2x80x9d bulk polymerisation method, or by chain extending prepolymers.
The general method of preparation of the novel polyurethanes is conventional and, as such, will be well known to those skilled in that art. In general, we prefer to use the prepolymer method because it provides good control over hard/soft segment proportions and over product quality.
In order to achieve good physical properties in polyurethanes, high molecular weights are needed. It has been recognised in the art that xcex1,xcfx89-hydroxy polyols sometimes contain monohydroxy terminated species (called xe2x80x9cmonolsxe2x80x9d) as impurities. These monols have only one hydroxy terminal and prevent the formation of high molecular weight products. The occurrence of monols in polyols can be reduced by using certain organometallic catalysts in the preparation, so that the polyol has only about 0.02 milliequivalents/g of unsaturation, but even this low level is not without effect. According to a feature of the present invention, we use xcex1,xcfx89-hydroxy polyols which contain no more than about 0.01, and most preferably no more than about 0.007, milliequivalents unsaturation per gram. Materials of this specification are available commercially. For example, Lyondell Chemical Co. (USA) supply xe2x80x9cAcclaimxe2x80x9d polyols which are said to have a very low level of monol impurity. These xe2x80x9cAcclaimxe2x80x9d polyols are for use in making high performance cast polyurethane elastomers to meet requirements not met by conventional rubbers and plastics, e.g. to provide high performance flexibility and toughness. Their utility in the present invention, in contributing to the production of polyurethanes closely matching natural rubber, is quite different from their proposed use for cast polyurethanes and, indeed, it is surprising that they are useful for the quite different purpose of the present invention.
Aliphatic diisocyanates are used to make the polyurethanes of the invention, since aromatic diisocyanates tend to give products of too high stiffness and creep to match natural rubber. Among the aliphatic diisocyanates which can be used is 4,4xe2x80x2-methylenebis(cyclohexyl isocyanate) (HMDI), available commercially as Desmodur W from Bayer. Others include isophorone diisocyanate (3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate IPDI), available from Veba; hexamethylene diisocyanate (HDI) available from Bayer; cyclohexane-1,4-diisocyanate (CHDI) available as a development product from AKZO; cyclohexane-1,4-bis(methylene isocyanate) (BDI) available as a development product from Eastman; 1,3-bis(isocyanatomethyl)cyclohexane (HXDI) available from Takeda; TMDI, a mixture of 1,6-diisocyanato-2,2,4,4-tetramethylhexane and 1,6-diisocyanato-2,3,4-trimethylhexane available from Veba; and the meta and para isomers of tetramethylxylene diisocyanate which are available as TMXDI from American Cyanamid.
The most preferred diisocyanates are alicyclic diisocyanates such as, for example, 4,4xe2x80x2-methylenebis(cyclohexyl isocyanate) available as Desmodur W.
Aliphatic diisocyanates generally form polyurethanes which are non-yellowing, and this is an advantage with soft thin walled elastic articles.
In general, the isocyanates used in the present invention are bifunctional (i.e. diisocyanates) and are of sufficient reactivity to give the desired high molecular weight polyurethanes, with the desired elasticity. The optimum choice of isocyanate will, of course, depend on the choice of polyol and of chain extender, and the proportions used, as will be clear to those skilled in the art. When butanediol is used as chain extender, we particularly prefer to use Desmodur W as the isocyanate.
The nature of the chain extender can vary quite widely. We prefer to use bifunctional compounds, i.e. diols, and suitable examples include ethane-1,2-diol, propane-1,2-diol, propane-1,3-diol, butane-1,4-diol, hexane-1,6-diol, 2,3-dimethylbutane-2,3-diol, 2,5-dimethylhexane-2,5-diol. A preferred chain extender is butane-1,4-diol. Where the polyurethane is to be used in aqueous dispersion, a chain extender containing a sulphonic acid group or carboxylic acid group, such as dimethylolpropanoic acid, is preferred. In addition, diamines and hydroxyamines may be used as chain extenders to generate poly(urethane-ureas). Examples of such chain extenders are ethylenediamine, propane-1,3-diamine, propane-1,2-diamine, butane-1,4-diamine, 2-methylpentane-1,5-diamine, hexane-1,6-diamine, ethanolamine, 1-aminopropan-2-ol, 3-aminopropan-1-ol, 2-aminopropan-1-ol, 2-aminobutan-1-ol and 4-aminobutan-1-ol. Use of amino compounds to prepare poly(urethane ureas) can have the drawback of restricting the solubility of the polymer to highly polar high boiling solvents such as dimethylformamide (DMF), N-methylpyrrolidone (NMP) and dimethyl sulphoxide (DMSO).
The proportions of isocyanate, and polyol and chain extender which are used will, of course, affect the proportions of hard and soft segments in the polyurethane, and the molecular weight. In general, the ratio hard:soft segment will be from 20:80 to 40:60.
The preferred method of preparation involves first reacting n mole equivalents of the polyol with n+1 mole equivalents of the diisocyanate to form a prepolymer. Thus, the weight ratios of polyol to diisocyanate will depend on the molecular weights of the reactants. By way of an example, if 1 mole equivalent of a polyol of molecular weight 2,000 is reacted with 2 mole equivalents of a diisocyanate such as HMDI of molecular weight 262, then the weight ratio of polyol to diisocyanate in the prepolymer is slightly less than 4 to 1. If the prepolymer is then extended with 1 mole equivalent of a diol, such as butanediol (molecular weight 90), the overall weight percentages of components in the final polyurethane will be 76% polyol, 20% diisocyanate, 4% extender. By way of a further example, if 2 mole equivalents of the same polyol are reacted with 3 mole equivalents of the same diisocyanate, the weight ratio of polyol to diisocyanate in the prepolymer will be approximately 5.1 to 1. If this prepolymer is now extended with 2 mole equivalents of the same diol and 1 mole equivalent of the same diisocyanate, then the overall weight percentages of components in the final polyurethane will also be 76% polyol, 20% diisocyanate and 4% extender.
In general, the proportions by weight of polyol, isocyanate and extender will be:
The number average molecular weights (Mn) of the polyurethanes of the invention will generally be in the range 90 to 150 kg/mole, preferably from 100 to 120. The spread of molecular weight as determined by the ratio Mw:Mn is generally from 1.2 to 2.2, preferably 1.4 to 1.8. (Mw is the weight average molecular weight.) As will be clear to those skilled in the art, the molecular weight of the polyurethane can be controlled by routine measures in the production process.
The polyurethanes of the present invention are closely matched to natural rubber in tensile properties. In this connection, the polyurethanes will generally have a S100 of less than 2.0 MPa, and most preferably less than 1.0 MPa, an elongation at break of at least 800%, and preferably above 1000%, and a tensile strength of above 15 MPa, and most preferably above 20 MPa. Polyurethanes with these particular combinations of properties so that they can be used in place of natural rubber, are novel.
Films of the invention are preferably made from organic solvent solutions or from aqueous dispersions of the polyurethanes, in conventional fashion such as by casting or dip coating. For solution fabrication, any suitable organic solvent can be used but we generally prefer to use tetrahydrofuran, although butan-2-one, xcex3-butyrolactone or aprotic solvents such as dimethyl formamide, dimethyl acetamide and N-methyl pyrrolidone may be used depending on the polyurethane structure. The aprotic solvents can be used in conjunction with a small amount of an inorganic halide such as 1 to 3% of lithium bromide. Alternatively, a small proportion of a non-solvent for the polyurethane may be used in conjunction with an appropriate solvent; these may include acetone, methyl isobutyl ketone, methylene chloride and chloroform, for example. The solutions can vary in the solids content but, for making articles such as gloves or condoms, the solids contents will usually be from about 10 to 30%, more usually from 15 to 20%. Films of the invention normally have a thickness of from 20 to 250 xcexcm, preferably 35 to 150 xcexcm for condoms and 100 to 200 xcexcm for gloves.
The production of polyurethanes with the unique combination of properties described is surprising. It would have been expected in the art that at the level of hard segment content used in the present invention, the resulting polyurethane would be much harder and of lower elongation at break than has actually been found. The combination of properties which are actually found is surprising and highly advantageous in its close match to the properties of natural rubber.
In order that the invention may be more fully understood, the following Examples are given by way of illustration only.